There is available a so-called class D amplifier as an audio power amplifier. The class D amplifier amplifies power by high-voltage/high-speed switching. Since the class D amplifier switches a supply voltage +VDD to generate an output voltage, radiation is generated by rising and falling edges of the output voltage. Further, the supply voltage +VDD is a high voltage of e.g., 20 to 50 V at the switching time, so that the radiation becomes significantly large.
In the case where the class D power amplifier as mentioned above is integrated with a receiver or disposed at the vicinity of a receiver, like the case of a car audio system, radiation generated by rising and falling edges of the output voltage may interfere with reception of e.g., AM broadcasting services.
As a technique to cope with the above problem, a class D amplifier described in, for example, Jpn. Pat. Appln. Laid-Open Publication No. 6-29757 was disclosed. In this class D amplifier, a carrier controller outputs a tuning signal having the frequency corresponding to tuner channel selection, as well as a carrier control signal for switching a time constant of a differential integrator. The carrier control signal switches an oscillating frequency of a closed-loop circuit, i.e., a switching frequency of a pulse amplifier. As a result, the frequency of the carrier signal of the closed-loop circuit does not interfere with the tuning signal of the tuner.
However, in the above class D amplifier disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 6-29757, a carrier frequency of switching amplification is changed according to a received frequency. As a result, it is difficult to perform a switching operation with high time axis precision using, e.g., a crystal oscillator, which may lead to deterioration in audio characteristics.